HOLOCENE COASTAL TERRACES ON THE SOUTHEAST COAST OF THE BOSO PENINSULA

Abstract

Along the southern coast of the Boso Peninsula, located about 80km south of Tokyo, have developed Holocene coastal terraces, which were elucidated to have been uplifted by tectonic movements associated with major earthquakes (Sugimura & Naruse, 1954; Matsuda et al., 1974). Some of these coastal terraces are composed of marine sediments, about 20m thick, whose facies changes reflect the fluctuation of sea-level relative to the land. The author has investigated these terraces to clarify the role of tectonic movements and eustatic changes of sea-level on their geomorphic development in the Holocene. The coastal terraces on the southeast coast of the Boso Peninsula can be divided into four surfaces by sharp cliffs, and have been named the Okaseda, the Teraniwa, the Hedate and the Setohama surfaces in the descending order of elevation. The Okaseda surface is distributed fragmentarily along the open coast; however, is widely found along the present river valleys as depositional surfaces of the Okaseda bed of the Holocene marine origin. The Okaseda bed is composed chiefly of bluish silt and fine sand which were deposited under the post-Glacial world-wide transgression. However, such changes in facies of this bed as alternations of silt, gravel and peat, intercalated in the lower Okaseda bed of bluish silt deposited about 9, 000 y. B. P., reflect minor oscillations of sea-level relative to the land, which can be interpreted to have been caused by mionr seismo-tectonic uplifts superposing on the eustatic rise of sea-level in the early Holocene. The Okaseda surface composed of the upper Okaseda bed of fine sand was formed at the time of the maximum rise of sea-level about 6, 000 y. B. P. The terrace surface is a flight of terracettes separated by minor scarps of 1 to 2m high, which is inferred to have been formed by a eustatic lowering succeeding to the maximum rise of sea-level and intermittent seismo-tectonic uplifts superposing on it. The Teraniwa surface develops continuously everywhere. Along the present river courses, the Teraniwa bed is composed mainly of marine sediments deposited in shallow valleys cutting the Okaseda bed, which indicates the submergence in this ares. Granting that this coast has been always uplifted by tectonic movements, the bed is inferred to have been deposited during a minor eustatic rise of sea-level. The Hedate surface is composed mainly of thin fluvial sand and gravel, and the Setohama surface is of erosional origin. The geomorphic history of this coast during the Holocene can be summarised as follows: (1) Since the rate of eustatic rise of sea-level was low about 9, 000 y. B. P., intermittent seismo-tectonic uplifts were reflected on changes in facies of the lower Okaseda bed. (2) From 8, 000 to 7, 000 y. B. P., valleys were drowned and filled with marine bluish silt, because the rate of eustatic rise of sea-level was rather high. (3) When sea-level reached to its highest level around 6, 000 y. B. P., the Okaseda surface was formed and the terracettes were developed by intermittent seismotectonic uplifts during the subsequent lowering of sea-level. (4) About 3, 500 y. B. P., a relative rise of sea-level (a minor eustatic rise of sea-level?) occurred and the Teraniwa bed was deposited in shallow valleys cutting the Okaseda surface. Soon sea-level turned to lower again and the Teraniwa surface emerged. (5) Since then, sea-level has continuously lowered, and the Hedate and the Setohama surfaces emerged by seismo-tectonic uplift.